In the case of access control systems such as those used, for example, in automotive engineering, it is necessary to design the systems in such a way that their power consumption is as low as possible. This applies both to a battery-operated mobile key which is carried by the driver and to the base station located in the automobile which is powered by the automobile's battery. The aforesaid components of an access control system generally have a microprocessor or controller. In order to save energy, such controllers are characterized in that they can be set to optionally different power-saving modes, in particular to a power-saving sleep mode in which even the clock unit of the controller is switched off (stop mode), and a quasi sleep mode in which, while the clock unit remains active, other internal and optionally external components of the controller are switched off or are run at low speed (low clock frequency) (pseudo stop mode).
Various systems in which this power-saving facility of controllers is used are known, for example, from patent application nos. DE-A-199 39 365 and WO-A-93/25987.
In order to save energy, it is also known for use to be made of a combination of a more powerful main controller having a correspondingly high power input in active mode and a less powerful pre-controller having a relatively low power input in active mode. The pre-controller is in this case kept constantly in active mode or pseudo stop mode so that it can perform any functions to be handled without any substantial time delay. For example, it is necessary in certain access control systems for the base station in the automobile to check periodically whether a mobile key has been activated in the area around the vehicle and whether in response to this transmit signal the vehicle's locking must be cancelled. To do this, the pre-controller can periodically activate the receiving unit of the base station, for example by applying the power supply voltage, and after waiting for the activation period (settling of filters, amplifiers and such like), sample the signal supplied by the receiving unit. If the pre-controller detects a received signal which has to be evaluated, for example regarding access authorization, then it can activate the main controller. The main controller, which is normally found in stop mode, has to be woken up for this purpose by means of an appropriate signal.
A base station of this type has the advantage, compared with a base station having only one correspondingly powerful main controller, of having a lower overall (mean) power input and a lower mean current input.
However, a disadvantage in a base station fashioned in this way is the relatively long response time, which is made up of the activation time in respect of the receiving unit and the activation time in respect of the main controller. Furthermore, the pre-controller means that there is an additional circuit-engineering outlay.